This invention relates generally to devices for keeping food serving dishware and food thereon warm prior to serving and, more particularly, to a heat storing plate warmer and a method for making said plate warmer.
Heat storing plates, or plate warmers, are commonly used in hotels institutional environments such as hospitals and nursing homes, and like operations to keep food warm prior to serving. Often the kitchen in such operations is far removed from the place where the food is served and consumed, and this, coupled with the large number of people to be served, frequently results in substantial delays between the time the food is removed from the oven and the time it is actually served. Such delays may, for example, commonly exceed thirty minutes by which time the food is cold. Accordingly, various plate warming devices for keeping food warm until it can be served, including several devices which are commercially available, have been suggested in the prior art.
One such device is disclosed in U.S. Pat. No. 3,557,774 wherein the device comprises a heat storage dish having an aluminum heat storage plate disposed in the space between the upper and lower walls of the dish. The metal heat storage plate is held against the bottom surface of the flat upper wall, and an insulating filler such as rock wool fills the remainder of the space between the heat storage plate and the bottom wall. In use, the heat storage dish is initially heated to store heat in the aluminum heat storage plate, and thereafter, when a plate of food is placed on the heat storage dish, the plate and the food are kept warm by the heat released from the heat storage plate. However, because air trapped in the space between the dish walls expands when the dish is heated, means must also be provided to relieve the internal pressure in the dish resulting from air expansion and thereby prevent the dish from bursting. Accordingly, the bottom wall of the heat storage dish shown in U.S. Pat. No. 3,557,774 includes an elevated annular wall portion which is deformable as the air in the space between the dish walls expands. One disadvantage of this dish, however, is that it requires a complex bottom wall having, in addition to the elevated annular wall portion, an outer circumferential reinforced support wall and a centrally disposed truncated conical wall portion extending upwardly from the annular wall portion through a central opening in the ring-shaped metal heat storage plate to attach to the top dish wall. Such an arrangement, of course, requires complicated fabrication and assembly and is not particularly suited for the mass production of such dishes.
The heat storage dish shown in U.S. Pat. No. 4,086,907 similarly utilizes a metal heat storage plate secured to the bottom surface of the flat top wall but eliminates the complex bottom wall described in the foregoing patent and substitutes indents or corrugations in the concave bottom plate wall which permit expansion or deformation of the bottom wall to prevent the dish from bursting should the plate be overheated. Because of the concave configuration of the base portion, however, the base itself provides substantial resistance to such expansion under normal conditions and makes the bottom of the plate relatively strong. This dish also suffers the disadvantage of requiring relatively complicated fabrication and assembly in that channel members extending through slots in the metal heat storage plate are spot welded to the top wall to secure the metal plate against the bottom side of the top wall. Moreover, such spot welds are also susceptible to breakage due to heat stress over continued periods of usage.
Another disadvantage of such dishes wherein metal heat storage plates are utilized is that because of the relatively high thermal conductivity of metals such as aluminum, the heat storage plate, when heated to a relatively low temperature, for example, 230.degree. F., is limited with respect to the amount of time it is effective to keep food warm. Although the heat storing dish may be initially heated to a relatively high temperature, i.e., in excess of 350.degree. F., to store sufficient energy in the metal heat storage plate to keep food warm for an appreciable period of time, this, of course, increases the inherent risk in handling such dishes and may cause the dish to burst. Also, while the heat storage plate can be increased in size to store more heat, the physical size and weight limitations for devices of this type generally do not permit increasing the size of the heat storage plate.
U.S. Pat. No. 3,148,676, on the other hand, discloses a food warming unit wherein the metal heat storage plate is replaced by a wax or asphalt substance having a relatively high specific heat and a relatively low melting point, e.g., between 180.degree. and 270.degree. F. The substance, which may comprise, for example, a wax such as carnauba wax, Cornox wax or a synthetic hardened microcrystalline wax, stores a relatively large amount of heat energy which is gradually released at a rate which is much less than the rate at which it was stored. The substance fills a chamber between the top and bottom walls of the unit and is retained within a honeycomb framework which is fabricated from aluminum or the like to form a multiplicity of relatively small, closely spaced cavities in the chamber. In this particular unit, expansion of the substance is accommodated by a pair of spaced circular lines of weakness in the annular recessed portion of the top wall which provide relief means for preventing the unit from bursting in the event that excess pressure is developed in the chamber. This unit, as do to the first two heat storage dishes described, also requires relatively complicated fabrication and assembly.
Further, fabrication of units utilizing heat storing substances, such as those disclosed in U.S. Pat. No. 3,148,676, is difficult because the heat storing substance is not readily insertable into the unit in its solidified state where, for example, a honeycomb framework or the like is required. If the substance is first melted for insertion into the honeycomb framework, the substance must be allowed to cool before further fabrication or assembly can be undertaken, and because one advantage of the heat storing substance is its capacity for heat retention for long periods of time, it is some time before the substance has cooled sufficiently to permit further work. Moreover, if the melted substance is injected into the device, the injection hole must be sealed, such as by soldering, and if the hole is improperly sealed, the seal may rupture due to expansion of the heat storing substance during use, allowing the substance to leak from device or allowing water or air to migrate into the chamber. In either case, mass production of such units is severely restricted.
Ideally, such devices should require only simplified fabrication and assembly, be low cost, and be effective to keep food warm for extended periods of time, i.e., more than thirty minutes. It is also desirable to provide such a unit wherein the walls of the unit do not have corrugations or the like, do not have complex surfaces, and are not weakened to provide pressure relief means.
Other heating devices similar to those discussed herein, but less relevant, are disclosed in the following U.S. Pat. Nos.: 780,352; 1,412,717; 2,640,478; 2,690,743; 2,791,204; 2,876,634; 3,164,148; 3,463,140; and 3,603,106.